Field of the Invention
The present invention relates to an inspection method performed in a process of manufacturing a gas sensor.
Description of the Background Art
Conventionally, various measurement devices have been used for recognizing a concentration of a desired gas component in a measurement gas. For example, as a device for measuring an NOx concentration in a measurement gas such as a combustion gas, known is a gas sensor (an NOx sensor) provided with a sensor element which is made of a ceramic having an oxygen-ion conductivity such as zirconia (ZrO2) (for example, refer to Japanese Patent Laid-Open No. 2006-284223). Such a sensor element of the gas sensor generally includes a plurality of electrode terminals on its surface to apply voltage, take out a detection signal, and supply electrical power to a heater (a resistance heating heater), for example.
In the meanwhile, a gas sensor includes a contact member into which the sensor element is inserted for holding the sensor element. For example, known is a gas sensor including a contact member which comprises a housing made of a ceramic (a ceramic housing) having a pair of housing members disposed to face each other to form an insertion opening into which a sensor element is inserted, a plurality of contact point members which are made up of a metal terminal to be attached to the housing members, and a plurality of lead wires connected to the contact point members for an electrical connection between the sensor element and outside (for example, refer to Japanese Patent No. 5082013).
In the gas sensor disclosed in Japanese Patent No. 5082013, the contact member holds the sensor element which is inserted into the insertion opening in a manner of contacting the contact point member and an electrode terminal, so that the electrical connection between the sensor element and the outside is obtained.
Moreover, also known is a method of manufacturing a circuit integrated gas sensor which prevents a disposal of a normal controller even when a sensor part has a defect (for example, refer to Japanese Patent Laid-Open No. 2007-225616).
In a technique disclosed in Japanese Patent Laid-Open No. 2007-225616, an inspection whether or not a detection element is normal is performed at the stage that a sensor intermediate assembly is formed before the detection element is integrated with the controller, and sensor intermediate assembly including an abnormal detection element is discarded, so that the expensive controller is prevented from being integrated with the abnormal detection element. In addition, the above inspection process is performed after confirming that a contact resistance of a lead terminal of the sensor intermediate assembly is at a level not having influence on an abnormality detection and a measurement of a characteristic information of the detection element, so that an accurate characteristic information for the detection element can be obtained, and a detection accuracy of an NOx sensor after completion can be secured.
In a case of a gas sensor disclosed in Japanese Patent No. 2006-284223, for example, the heater included in the sensor element is used for maintaining an operation temperature of the sensor element within a predetermined level. Since the operation characteristic of the sensor element varies depending on the temperature, an electrical resistance value of the heater (referred to as the heater resistance value) is one of the important characteristic values for the gas sensor. Therefore, for a mass-produced gas sensor, required is a securement that the heater resistance value falls within a predetermined range determined as a standard. Accordingly, an inspection whether or not the heater resistance value meets the standard is performed in the process of manufacturing the gas sensor.
However, only the evaluation of the heater resistance value on the sensor element itself before incorporated into the gas sensor is not preferable even when the heater resistance value of the sensor element can be accurately measured by reason that even when a defect occurs in an assembly process, the defect is not detected in the gas sensor which is finally obtained.
In the meanwhile, if only a gas sensor after assembled is subject to the inspection, the sensor element whose heater resistance value does not meet the standard is also provided to the assembly of the completed gas sensor, so that it is not preferable in point of productivity.
Accordingly, it is preferable to perform a two-stage inspection for the heater resistance value, that is, the inspection of the sensor element before incorporated into the gas sensor and the inspection of the gas sensor after assembled.
However, as disclosed in Japanese Patent No. 5082013, for example, in the gas sensor after assembled, the electrode terminal of the sensor element is electrically connected to the contact point member of the contact member and is not exposed to the outside in general. Accordingly, the inspection for the heater resistance value of the gas sensor after assembled needs to be performed via a lead wire connected to the contact member or via a connector, to which the lead wire is connected, for connecting an external equipment, differing from the inspection before incorporation which can be performed by directly accessing the electrode terminal of the sensor element.
The inventor of the present invention obtained a finding, as a result of earnest review, that when the heater resistance value is inspected, a determination of the presence or absence of an assembly defect of a gas sensor is enabled, more specifically, a determination of the presence or absence of a contact abnormality between the electrode terminal of the sensor element and the contact point member of the contact member is enabled, using the above difference between the subjects for the measurement, in addition to a determination whether or not the heater resistance value meets a predetermined standard.
Exemplified as a cause of the contact abnormality is that, for example, particles of talc (an insulating ceramic powder) which airtightly seal the sensor element in the gas sensor are sandwiched (caught) between the electrode terminal of the sensor element and the contact point member of the contact member, or that the electrode terminal is peeled out.
In the mass production process of the gas sensor, a large number of sensor elements are manufactured in one lot at the same time and each of them are provided to the assembly of the gas sensor, and a gas sensor whose heater resistance value does not meet the standard or a gas sensor which has the assembly defect needs to be reliably excluded from a shipping object.
Although Japanese Patent No. 2007-225616 discloses that the characteristic information is obtained by a performance inspection of the intermediate assembly, it does not disclose or suggest the inspection of the assembly state of the gas sensor using the heater resistance value.